This invention relates to optical fibers and, more particularly, to a method for optically monitoring the properties of plastic coatings as they are applied to optical fibers.
Plastic coatings that are applied to optical fibers serve multiple purposes. When applied as an overcoat to glass-clad optical fibers, they serve to reduce microbending loss, maintain the pristine strength of the fibers and provide for abrasion and mechanical protection of the fibers during cable manufacturing processes. In addition, the plastic coated fiber may decrease crosstalk between optical fibers and improve the long term stability of fibers in an uncontrolled environment. The plastic coating can also be directly used as the cladding for fused silica cores since the refractive index of many polymer materials is less than that of fused silica.
For optimum performance the coating must be applied uniformly and concentrically around the fiber. This is necessary for routine handling and splicing of the fibers as well as for optimum strength and transmission characteristics.
The coatings which include such materials as silicon, epoxy-acrylates, tetrafluoroethylene, perfluoro-vinyl-methyl ether, perfluoronated-ethylenepropylene, and ethylene-vinyl-acetate copolymer, are applied to fibers by various methods. Prior art techniques for examining the concentricity of coatings once they are produced require the microscopic examination of the fiber after a run has been processed. In addition to being time-consuming and destructive, this prior art procedure may not detect beading or gross nonuniformities which could seriously impair the transmission characteristics of the fiber. More importantly, real time information to enable the fabricator to make corrections, evaluate various applicators or stop the process completely is not available as the coating is being applied.
In U.S. Pat. No. 3,879,128, issued Apr. 22, 1975, to the present inventor, the evaluation of various parameters of an optical fiber by examining the backscattered light produced in response to an incident beam is disclosed. The disclosed method, however, cannot determine the concentricity and uniformity of application of plastic coatings on an optical fiber.